Electron Paramagnetic Resonance (EPR) provides a sensitive means of detecting and quantitating free radical species. Conventional constant wave (CW) EPR has provided insight into the basic chemistry of free radical reactions and has most recently been increasingly used to probe the intricacies of biological intermediates. At this time, although EPR is vastly more sensitive than nuclear magnetic resonance, its use as a biological tool has been limited by both the frequency used--routinely greater 1 MHz--and the paucity of detectable signal. Because radiation induces a cascade of electrons and ultimately results in formation of free radical species, the ROB has been developing both a pulse wave (PW) EPR spectrometer and Electron Paramagnetic Imaging (EPI). Since excited electron relax in microseconds and less, the use of nanosecond PW-EPR with heretofore unachievable rapid signal averaging and processing is required for detection of transient, short-lived free radial signals. Likewise, the previous technologic limitations imposed by CW-EPR microwave frequencies are not an impasse when EPT with pulsing techniques and gradient profiling and sophisticated electronics and computational techniques are employed. The goals of this project are to create a prototypical PW-EPR instrument for in vitro biologic studies and to establish the foundation of PW-EPI in vivo imaging. The first priority of the project to demonstrate at radio frequencies one and two dimension images of electrons is completed. The second milestone of developing an intermediately fast averager has been completed and has allowed detection of the first low frequency, pulsed EPR signal of a spin label (nitroxide; T1/2 = 90 nanoseconds). The initial successes assure feasibility. Now the limitations of the concepts and electronics and computation are being reworked.